This invention relates to a control apparatus for an A.C. elevator which controls the elevator that is driven by an induction motor.
FIGS. 4 and 5 are connection diagrams of such a prior-art control apparatus for an A.C. elevator as disclosed in Japanese patent application Laid-open No. 59-7679. Referring to FIG. 4, the control apparatus includes a three-phase A.C. power source 1, a converter 2 which is composed of diodes or thyristors, a smoothing capacitor 3 which is connected on the D.C. side of the converter 2 to smooth the D.C. power thereof, an inverter 4 which inverts the D.C. output of the smoothing capacitor 3 into A.C. power, a three-phase induction motor 5 which is driven by the A.C. power of the inverter 4, a brake wheel 6 which is mechanically coupled to the three-phase induction motor 5, and a brake shoe 7 which is disposed in opposition to the side peripheral surface of the brake wheel 6 and which applies a braking force to the brake wheel 6 by virtue of the force of a spring 8. A brake coil 9 draws the energized brake shoe 7 away from the brake wheel 6 against the force of the spring 8, and a D.C. power source 10 supplies D.C. power to the brake coil 9. The brake wheel 6 and the D.C. power source 10 mentioned above shall hereinbelow be simply called the "brake."
A driving sheave 11 is coupled to the three-phase induction motor 5 through the brake wheel 6 and is hoisted by the drive of the motor, while a main rope 12 is wound round the driving sheave 11 and has a cage 13 and a counterweight 14 coupled to both its ends. A tachometer generator 15 is directly coupled to the three-phase induction motor 5, and generates a speed signal Va expressive of the revolution speed of the motor. A control device 17 generates ignition signals 17a-17f for subjecting the output of the inverter 4 to a variable-voltage variable-frequency control, namely, for controlling the voltage and frequency thereof, on the basis of a speed command signal Vc and the speed signal Va. An abnormality detection device 18 detects the abnormality of the control device 17 on the basis of the speed signal Va and the speed command signal Vc. An abnormality detecting relay 19 has a normally-open contact 19a, and it is in an energized state during normalcy and is brought into a deenergized state by the abnormality detection signal of the abnormality detection device 18.
Referring to FIG. 5, a braking electromagnetic contactor 21 has one end connected to the plus (+) terminal of the power source through the normally-open contact 19a as well as a start command relay contact 22 which is closed by a start command delivered from an elevator controller not shown, while it has the other end connected to the minus-terminal of the power source. The contractor 21 further has a contact 21a which is connected between the brake coil 9 and the D.C. power source 10 shown in FIG. 4. Likewise to the braking electromagnetic contactor 21, an operating electromagnetic contactor 23 has one end connected to the plus + terminal of the power source through the normally-open contact 19a as well as another start command relay contact 22, while it has the other end connected to the minus-terminal of the power source. It has a normally-open contact 23a which is connected between the three-phase A.C. power source 1 and the converter 2.
FIG. 6 is a block diagram of the control device 17. Referring to the figure, an adder 17g evaluates the deviation (signal) between the speed command signal Vc and the actual speed signal Va. A speed controller 17h corrects the deviation from the adder 17g, and provides a slip frequency command 17i equivalent to a torque command. A voltage command generator 17j receives the slip frequency command 17i and the speed signal Va, and generates an A.C. voltage command. A pulse width modulator 17k generates the controlled ignition signals 17a-17f in accordance with the A.C. voltage command received from the voltage command generator 17j, and applies these ignition signals 17a-17f to the inverter 4 so as to subject it to the variable-voltage variable-frequency control.
Next, the operation of the prior-art apparatus will be described. During the stop of the cage 13, the brake shoe 7 is kept depressed on the brake wheel 6 by the force of the spring 8. Meanwhile, the abnormality detection device 18 holds the abnormality detecting relay 19 in the energized state to close the normally-open contact 19a when the deviation between the speed command signal Vc and the speed signal Va does not exceed a preset value, that is, when the control device 17 is not abnormal. In addition, when the start command of the cage 13 has been issued, the start command relay contact 22 is closed, and hence, the operating electromagnetic contactor 23 is energized to close the normally-open contact 23a thereof. Accordingly, the three-phase A.C. power is fed to the converter 2 through the normally-open contact 23a, and the converter 2 starts the delivery of the D.C. power. In response to the D.C. power, the inverter 4 supplies the three-phase induction motor 5 with the three-phase A.C. power of variable voltage and variable frequency which correspond to the rotating direction of the three-phase induction motor 5.
Since the start command relay contact 22 is closed by the start command of the cage 13, the braking electromagnetic contactor 21 is energized to close the contact 21a thereof. Accordingly, the brake coil 9 is energized by the D.C. power source 10, the brake shoe 7 is drawn away from the brake shoe 6, and the brake is released. Therefore, the three-phase induction motor 5 starts rotating in accordance with the three-phase A.C. power supplied from the inverter 4 and runs the cage 13.
The rotating speed of the three-phase induction motor 5 is detected and turned into the speed signal Va by the tachometer generator 15. Therefore, the control device 17 controls the ignition signals 17a-17f in accordance with this speed signal Va and the speed command signal Vc, thereby to control the voltage and frequency of the three-phase A.C. power from the inverter 4 toward the three-phase induction motor 5. That is, the control device 17 controls the rotating speed of the three-phase induction motor 5, accordingly the running speed of the cage 13, into a value corresponding to the speed command signal Vc. When the cage 13 has approached a floor to stop on, owing to the running thereof, the control device 17 begins a deceleration control, through which the cage is stopped at the position of the floor. Under this state, the start command relay contact 22 is opened, and the operating electromagnetic contactor 23 is deenergized, so that the normally-open contact 23a is opened. Also, the braking electromagnetic contactor 21 is deenergized, the contact 21a thereof is opened, and the brake coil 9 is deenergized. Finally, the brake shoe 7 is depressed on the brake wheel 6, and the cage 13 is stopped.
If, due to any abnormality, the deviation between the speed command signal Vc and the speed signal Va has become greater than the preset value after the issue of the start command of the cage 13, the abnormality detection device 18 produces the abnormality detection signal to deenergize the abnormality detecting relay 19 and to open the normally-open contact 19a thereof. Accordingly, the inverter 4 ceases the supply of the three-phase A.C. power to the three-phase induction motor 5. Moreover, since the brake coil 9 is deenergized in a sequence reverse to the foregoing, the brake shoe 7 frictionally brakes the brake wheel 6 so as to stop the cage 13.
The prior-art control apparatus for an A.C. elevator is constructed and operated as described above. Therefore, in a case where, in the state in which the brake shoe is depressed on the brake wheel by a cause such as the strong exertion of the spring 8 or the disconnection of the brake coil at the start of the cage or during the running thereof, the cage is operated downwards with a load near a rated carrying capacity or is operated upwards with no load, it can occur that the deviation between the speed command signal and the speed signal becomes smaller than the preset value for the abnormality detection and that the abnormality detection device fails to respond, and this poses the problem that a situation dangerous as the elevator arises. In such a case, the cage runs with the brake shoe actuated, so that a brake lining mounted on the brake shoe is heated and abraded until the braking function of the brake shoe degrades at last.
The cause by which the abnormality detection device fails to respond in the running mode of the cage, is as follows: In the case of the above operating mode performed with the brake released, a torque required for the acceleration of the induction motor is small. Therefore, even in the state in which the brake works, the induction motor can develop a torque required for executing the above operation, and the deviation between the speed command signal and the speed signal does not become large enough to actuate the abnormality detection.